Active matrix liquid crystal display devices are well-known throughout the art. For example, U.S. Pat. Nos. 4,855,724; 3,862,360; 4,112,333; and 4,762,398 disclose active matrix liquid crystal displays having row and column address lines and corresponding driving circuitry. This type of display is commonly known throughout the art as an "X-Y active matrix type LCD". In such displays, both the row and column address lines disposed in the display panel are generally interdigitated (one-half extending to one edge of the display panel and the other half to the opposite edge) so that the address lines can be driven from all four peripheral sides of the panel. Such displays have many applications including uses in air vehicle cockpits, AMLCD projectors, televisions, lap-top computer screens, etc.
Conventional packaging arrangements in X-Y active matrix LCDs include mounting the row and column address line driver chips on separate rigid printed circuit boards disposed adjacent the peripheral edges (i.e. sides) of the display panel and interfacing the driver chips with a central controller for controlling the output of the AMLCD, each edge of the display panel having a separate circuit board mounted adjacent thereto. This type of AMLCD assembly is disclosed, for example, in U.S. Pat. Nos. 4,772,100 and 5,155,612. In these displays, the address lines extending to a particular side of the display panel are interfaced with the circuit board corresponding to that side of the display panel. This, unfortunately, results in the need for up to four different circuit boards, one for each side of the display panel having exposed address lines. These circuit boards typically extend laterally with respect to the plane of the display panel and occupy valuable boarder space.
In aforesaid U.S. Pat. No. 4,772,100, the circuit boards are mounted below the display panel so as to conserve boarder space. This design, however, significantly increases the overall depth of the AMLCD assembly rendering it difficult to install in areas having strict depth requirements, such as avionic cockpits.
It is therefore desireable to mount the driver chips on the display panel via flexible circuit elements, to which the chips are attached. The flexible circuit elements are orthogonally bent around the peripheral edges of the panel to optimize space. The row and column address lines of the display panel are electrically connected to one end of the flexible circuits and thereafter interfaced with the central controller by way of the driver TABS, with a circuit board corresponding to each side of the display being disposed between the flexible circuit elements and the central controller. Unfortunately, these AMLCDs still require up to four different circuit boards to interface all of the display panel address lines with the controller.
U.S. Pat. No. 4,836,651 discloses such a flexible circuit assembly for an active matrix liquid crystal display. FIG. 1 is a perspective view of this prior art AMLCD (active matrix liquid crystal display) assembly including multi-layer flex circuits 11 and 14 interconnecting the row and column panel address lines (not shown) and the drivers 12 and 13 disposed on the flex circuits. For simplicity purposes of illustration, the housing in which the display panel, backlight, and associated circuitry are mounted is not shown in FIG. 1.
The prior art active matrix liquid crystal display panel of FIG. 1 is illustrated generally at 10 and typically includes of a pair of opposing transparent glass substrates sealed peripherally so as to define a planar cavity in which a liquid crystal layer is retained. The interior surface of one of the substrates has a transparent ground plane electrode (not shown), preferably of indium tin oxide (ITO), disposed thereon. Deposited on the interior surface of the other glass substrate is a pattern of individual transparent pixel electrodes (not shown) which define the X-Y matrix of individual liquid crystal cells. These cells form the array of liquid crystal picture elements or pixels. The individual pixels are separated by X and Y directed address lines so as to form a row and column arrangement with the address lines being interdigitated so that they may be driven from all four peripheral edges of the display.
Shown affixed to the front or viewing side planar surface of the display panel 10 and orthogonally bent around the panel peripheral edges are a plurality of multi-layer flex circuits 11 and 14 which support row drivers 12 and column drivers 13, respectively. The row and column drivers 12 and 13 supported on the multi-layer flex circuits 11 and 14 include hermetically sealed leadless chip carriers which contain driver chips (not shown).
One longitudinal end of each of the multi-layer flex circuits 11 and 14 is connected to the individual address lines (row or column) extending to the edge of the display panel 10. When the flex connectors are soldered to these address lines, the flex connectors are, as shown in FIG. 1, bent orthogonally relative to the plane of the liquid crystal display panel 10 thereby minimizing the circumferential boarder extent of the display assembly. However, the disposition of the drivers 12 and 13 at the ends of the flex circuits 11 and 14 significantly increases the depth of this AMLCD assembly.
In a typical 4.times.4 active matrix liquid crystal display having a line density of 100 lines per inch, each of the four sides of the display has 200 address lines driven therefrom. Thus, each of the flex circuits shown in FIG. 1 would drive 50 address lines. Flex circuits 11 and 14 are respectively connected to cascade circuit boards 15 and 16 and thence to multi-pin connectors 17 and 18 for energizing the driver circuits. The multi-pin main connectors 17 and 18 allow the driver chips and flex circuits affixed thereto to interface with a conventional central controller for selectively controlling the output of the display.
In a typical operation of the aforesaid prior art AMLCD, the central controller (not shown) outputs data signals for selectively energizing certain pixels of the active matrix array. The signals from the controller proceed through main connectors 17 and 18, and are input through the circuit boards 15 and 16 to the driver chips disposed in drivers 12 and 13. The driver chips then provide output signals through their respective flex circuits 12 and 13 to predetermined address lines for activating particular pixels in the matrix array.
While the driving circuitry of FIG. 1 reduces the overall peripheral size of the prior art AMLCD, the design is not without its drawbacks. The AMLCD requires four separate cascades 15 and 16 as well as separate flexes 11 and 14 to be mounted adjacent each of the four peripheral sides of the display panel 10. The presence of so many circuit elements significantly adds to the cost of manufacturing this AMLCD assembly. Furthermore, the disposition of the cascade circuit boards structurally between the drivers and the main connectors increases the depth of the display assembly. Also, each circuit board requires its own main connector (e.g. 17, 18) for the drive circuitry of each peripheral side to be interfaced with the central controller, thus increasing the cost and complexity of the assembly.
It is known to connect adjacent circuit boards to one another using flexible circuits. In other words, four different such circuits could be utilized in the AMLCD assembly of FIG. 1, each one connected between adjacent circuit boards. This, however, again requires four separate circuit boards and four separate flexes.
It is also known to wrap a flex circuit around the periphery of a display panel. See "Meeting the Challenges of Flexible Circuits," Printed Circuit Design, July, 1992. A significant drawback of such a design is that only a small number of conductive trace layers may be disposed on the flexible circuit due to the fact that the circuit is non-planar through its entirety and includes up to four 90 degree bends therein. Therefore, this type of design is only useful in very small displays where few traces are required for addressing the panel. The aforesaid flex circuit design thus cannot be used to address high resolution AMLCDs requiring an increased number of trace layers due to its wrap-around design which significantly limits the number of traces (e.g. copper) which may be disposed at the corner or bend areas of the circuit.
Another known manner in which to reduce the overall size of AMLCD assemblies involves attaching individual driver TABS (TAB=tape automated bonding; hereinafter "TAB") along each of the four peripheral sides of an AMLCD panel and providing a separate rigid circuit board adjacent each side of the display. Each such circuit board thus interfaces the central controller with the driver TABS mounted along the corresponding side of the display panel.
FIG. 2 illustrates such a driver TAB which was offered for sale more than one year before the filing date of this application. This TAB is typically mounted along an edge of an active matrix display panel so that the address lines of the active matrix panel are electrically connected (e.g. soldered) to the output leads 29 of driver TAB 20. Window 25 in base portion 21 is provided so that output lead support 32 may be mounted to the viewer side planar surface of the display panel and base portion 21 can be bent orthogonally (i.e. about 90.degree.) with respect thereto around the peripheral edge or side of the panel. Therefore, the portions 28 of output traces 29 crossing window 25 are bent about 90.degree. so that base 21 extends orthogonally from the rear planar surface of the display panel. The input traces 31 of TAB 20 are interfaced with the central display output controller (not shown) by way of input contacts 34 (i.e. leads) defining the driver TAB input contact row or input pad.
Driver TAB 20 includes a uni-layer base portion 21, preferably made of a polyimide, upon which the plurality of copper input and output traces 31 and 29 are disposed, preferably by etching. Base portion 21 defines a horizontally aligned contact row window 23 over which input traces 31 cross to create the input contact row. A driver chip or die 27 is affixed by way of gold welds to the input and output traces disposed on base portion 21. After chip 27 is gold-welded, it is encapsulated so as to seal the gold welds along with the chip itself. TAB input traces 31 electrically interface the driver chip or die 27 with the central display output controller. Tooling holes 33 are defined by the base portion 21 so as to allow the driver TAB 20 to be securely aligned and mounted during its manufacturing and trimming processes.
Typically, input signals for selectively controlling the activation of individual pixels in the matrix array originate at the central display output controller and make their way to driver chip 27 by way of the input contact row (i.e. input pad) 34. The input signals proceed from input pad 34 to driver chip 27 via input traces 31, the chip in turn dictating the output signals sent to the display panel through output leads 29. The output contact row 29 disposed on portion 32 electrically interfaces driver chip 27 with the display panel address lines. Accordingly, each of the output leads 29 is connected to either a row or column address line of the active matrix by conventional means thereby enabling the output of the display panel to be controlled by the signals sent from the central controller.
It is to be noted that certain input contacts (e.g. 35 and 36) are common leads which do not interface with driver chip 27. These leads are, for example, connected to the common plane of the display panel or may represent conventional replacement leads.
As will be appreciated by those of skill in the art, the aforesaid prior art driver TAB can only accommodate a predetermined number of input and output leads dictated by the overall width of base portion 21 (or longitudinal length of the input contact row). If more such leads are required to interface the display output controller with the address lines of the active matrix display panel, more such driver TABS must be used or the overall size of the TABS must be increased. Additional leads/traces cannot be disposed on the base portion 21 of driver TAB 20 because the pitch between the contacts must remain large enough so as to enable good electrical connection via soldering or welding between the input contacts (i.e. leads) of driver TAB 20 and their corresponding connecting lines.
It is known to provide such TABS with two vertical input contact rows (input pads) thus allowing the TAB to mount more leads, the term "vertical" meaning that the longitudinal axis of each such row is substantially perpendicular to the plane of the display panel. This, however, significantly increases the depth (or height) of the TAB which in turn adds to the overall depth of the display assembly.
Accordingly, it would satisfy a long felt need in the art if a driver TAB having dimensions substantially similar to driver TAB 20 could be designed to accommodate significantly more input and output leads without significantly decreasing the pitch of the contacts in each row or pad, thereby enabling more AMLCD address
It is apparent from the above that there exists a need in the art for a high resolution active matrix liquid crystal display assembly of the X-Y matrix type where all of the address lines of the display panel are interfaced to the display output controller by a single flex circuit thereby eliminating the need for multiple circuit boards, connectors, and flexes. Such a flex circuit should be able to accomodate an unlimited number of conductive trace layers so as to be able to address AMLCDs of varying resolutions. There also exists a need in the art for a driver TAB which can drive an increased number of AMLCD address lines without increasing the size of the display assembly.